The invention relates generally to instrument cables for use in elongated passages, and more particularly concerns a reduced diameter cable for use in high pressure environments.
There has long been a need to visually examine down-hole conditions of a well for various reasons. One of the more common uses of down-hole video is leak detection. The camera system may detect turbulence created by a leak and may identify different fluids leaking into the well bore. Particulate matter flowing out through a hole can be detected. Damaged, parted, or collapsed tubings and casings may also be detected. The severity of scale buildup in downhole tubulars, flow control devices, perforations and locking recesses in landing nipples can be seen and analyzed.
Additional uses for video camera systems include the detection of formation fractures and their orientations. Video logging provides visual images of the size and extent of such fractures. Down-hole video is also useful in identifying down-hole fish and can shorten the fishing job. Plugged perforations can be detected as well as the flow through those perforations while the well is flowing or while liquids or gases are injected through the perforations. Corrosion surveys can be performed with down-hole video and real-time viewing with video images can identify causes for loss of production, such as sand bridges, fluid invasion, or malfunctioning down-hole flow controls.
Down-hole instrument probes can be made extremely small due to the existence of charge-coupled device imaging systems and other technologies which can function as a camera in the down-hole instrument. Electrical circuitry inside such an instrument can also be made small with the use of semi-conductor devices. The instrument probe containing the remote video camera system and other electrical equipment is connected to the surface equipment by an umbilical instrument cable thereby permitting transmission of electrical power to the video camera and communication of data from the video camera to the surface equipment.
Many wells are relatively small in diameter, on the order of 4.5 cm (1.75 in). Consequently the instrument probe and its cable designated for use in such a well are limited in their respective diameters. This can lead to practical problems when a high pressure well is involved. Such wells are capped to prevent the uncontrolled escape of high pressure well fluids and, in order to insert a down-hole video instrument in such a well, the instrument must be forced into the well through the cap. As is well known in the art, smaller instruments are easier to insert into a high pressure environment because they present less surface area against which the high pressure well fluids can act. Those high pressure well fluids oppose entry of the cable into the well and the cable must be made heavy enough to overcome that fluid pressure force. Also, it has been found that small differences in the diameters of down-hole instrument cables can have a tremendous impact on the ease and expense in inserting that cable and an attached instrument into the well.
Referring now to the graph of FIG. 1, it can be seen that at a well pressure of 281 kg/cm.sup.2 (4000 psi), a cable with a 1.11 cm (7/16 in (0.438)) diameter will require the addition of 295 kg (650 lbs) additional weight to overcome the force against it created by the well fluid pressure to enter the well. One common technique for adding that weight is to attach sinker bars to the cable. The diameter of the well limits the diameter of the sinker bars requiring a longitudinal distribution of the weight along the cable. In a 4.5 cm (1.75 in) diameter well, sinker bars having the standard outside diameter of 3.5 cm (1.375 in) would be used. Even if using the high density tungsten weights, each bar would be 1.8 meters (6 ft) long and has a weight of only 20.4 kg (45 lbs). This would result in the need for 15 sinker bars placed end to end on the cable and at 1.8 m (6 ft) each, a total length of 27.4 m (90 ft) of sinker bars results. Adding this length to the length of the instrument itself, which may be 4.5 m (15 ft), a total length of 31.9 m (105 ft) exists for the complete assembly. As shown in FIG. 2, the cable 16 must be raised above the well head 2, inserted through a pressure gland 4, through lubricator risers 6, and past the main valve 8. In this case with such a long length of weights, an extended crane would be required to lift the assembly of instrument, cable, and sinker bars over the main valve 8 of the well head 2 and the specially attached lubricator risers 6 attached to the well head as shown in FIG. 2 to accommodate the assembly. It has been found in some cases that the expense involved in supporting such a long length of lubricator risers 6, the need for high crane heights, and the amount of time involved in assembling and disassembling outweigh the advantage that would be provided by down-hole video.
A further review of FIG. 1 shows that for a cable having a diameter of 0.55 cm (7/32 in (0.218)) (approximately half of the previous cable diameter) and in a well having the same pressure of 281 kg/cm.sup.2 (4000 psi), the weight required to overcome the fluid pressure and insert the cable into the well is only 77 kg (170 pounds), which is approximately one-fourth of the weight required for a cable twice its size. Using the same tungsten weight bars as described above, only four are required and at 1.8 m (6 ft) each, the total length of the lubricating risers needed to accommodate the weights and the instrument is 12 meters (39 ft). This is much more practical and much less expensive than the length required in the previous example. As is apparent from FIG. 1, even small changes in cable diameter result in much larger changes in weight requirements. Hence those concerned with high pressure wells have recognized the substantial effect that cable diameter has and have recognized the need for a reduced diameter cable so that insertion into high pressure wells is facilitated and made less expensive.
Another consideration in cable design is the impact of the cable length on the size of the internal cable components. In the case of a coaxial cable, the longer the cable, the larger the cable diameter must be to support needed data transmission parameters for real-time video. Thus, it has been found that for a coaxial cable length of 4,572 m (15,000 ft), a cable diameter of 1.3 cm (0.52 in) is needed to obtain the data rates desired for real time video. As shown above, this diameter cable results in an impractical length of weights for higher well pressures. However, it has been found that optical fibers are not as sensitive to long distances and have large bandwidths capable of supporting real-time video imaging. The use of fiber optics enables use of a much smaller diameter cable.
It is also important for a down-hole instrument cable to include electrical conductors for the conduction of electrical energy such as power. Electrical conductors also take up space in a cable and therefore it has been recognized that the electrical conductors should also be kept to as small a size as possible. However, certain electrical performance requirements must still be met. Additionally, the conditions within a well to which the instrument cable is exposed can be quite harsh, with hydrostatic well pressures in excess of 421 kg/cm.sup.2 (6,000 psi) and ambient wall temperatures reaching 110.degree. C. (230.degree. F.) and higher. Wells may contain certain caustic fluids such as hydrogen sulfide which can cause optical fiber deterioration and poor performance. The fiber must be protected from leakage of such fluids. Wells often also have joints with protruding collars against which the cable can rub while the cable is inserted and withdrawn from the well. Sharp objects in the well can also damage the cable, and can break through and severely damage a fluid seal provided by an outer sheath of plastic on cable so equipped. Therefore, it has been recognized by those skilled in the art that a fluid tight seal is needed about the optical fiber or fibers in the cable as well as a rugged outer cable surface.
In many cases, the well can also be quite deep, and the length of the down-hole instrument cable can exceed 4,572 to 4,877 meters (15,000 to 16,000 ft). Longitudinal stresses placed on an optical fiber in such a long cable can sever or fracture the optical fiber, causing significant signal attenuation. Hence, the cable must be designed not only to resist physical damage to its outer surface from use in the well, and provide a robust fluid seal to protect the optical fiber and electrical conductors, but also to support the weight of the down-hole instrument and the cable itself.
Down-hole optical fiber instruments include terminations for receiving the optical fiber, electrical conductors, and strength members. Such connections should be implemented in a way such that the internal components of the instrument probe are isolated from the high pressures and temperatures within the well bore.
Hence those skilled in the art of down-hole instrument cables and terminations have recognized the need for a reduced diameter cable for use in high pressure wells and terminations made in a way which does not subject the instrument to the high pressure fluids of the well. The present invention satisfies these needs and others.